Present-day demands to control temperatures generate a high interest in flexible electric heating devices for ensuring a temperature parameter. Cloth heaters are a type of flexible electric heating means for heating various surfaces and media that is easily adaptable to an application site. Among the devices that require electric heating cloths are automotive heated seats, heated steering wheels, automotive engine oil crankcases, cement hardening heaters, clothes with heating elements, thermal blankets, etc.
Simple flexible electric heaters are usually comprised of thin metallic electric heating wires connected to a flexible surface in a serpentine shape. The electric energy supplied to the heating wires is dissipated, thus dissipating the heat from the wire into the environment. The insulating material that covers the heating wires from both sides ensures electric insulation and heat propagation. However, heat generation with heating wires has so far resulted in non-uniform heating, which is especially noticeable in devices in which the heater is installed in close proximity to a person. Besides, metallic heating elements are susceptible to bending and twisting and have a tendency to break. Also, the maximum heating temperature range is limited by the heating wire gauge. Application of simple cloth electric heaters is an attempt to alleviate the problems of flexible electric heaters by using multiple conducting threads interweaved in the cloth, as heating elements. The cloth is a combination of heating conducting threads of a “shell-nucleus” type that run in one direction, and primary threads that run in another direction. The electric energy is supplied to the heating threads with the help of low-resistance conducting threads along the cloth edges that are arranged perpendicular to the heating threads in the cloth.
Cloth heaters alleviate the problem of non-uniform heating by use of multiple parallel heating threads connected with each other by conducting bus bars.
In some applications, such as automotive seat heaters, it is desirable that a constant operating heating temperature of approximately 37° C. be maintained, with provisions to raise it to about 150° C. within a short period of time during fabrication of seats in order to ensure melting of the adhesive material that bonds of the car seat material to the foam.
Introduction of modern composite materials that can withstand reasonably high temperatures made it possible to produce devices that are not susceptible to limitations on the maximum heating temperature within a permissible range. Use, in the cloth, of “shell-nucleus” type heating resistive threads produced with the help of known processes does not make it possible to expand the temperature range as required, due to reasons that will be described below.
For example, from WO, 95/17800, HO5B 3/36, publ. Jun. 29, 1995, an electric heating woven thread cloth is known that has 0.3–3.5 kOhm/m linear electric resistive heating threads both in the weft and the warp. The heating resistive thread used in the known cloth has a “shell-nucleus” structure, in which the “nucleus” consists of polycaproamide fiber, and the “shell” that serves as the resistive material is a composite that includes a tetrafluoroethylene co-polymer with vinylidene fluoride and industrial carbon. The heating resistive thread is produced by applying a coat of resistive material on polycaproamide fiber.
The disadvantages of this heating resistive thread are: low linear electric resistance, which limits its use to fabrication of woven heating elements that are suitable for work at voltages not exceeding 36 V; use of polycaproamide fiber of a certain configuration, only, as a “shell” for its production; and higher resistive material consumption.
Besides, the heating temperature of the woven heating element, produced using this thread as a basis, cannot exceed the polycaproamide fiber melting point (100–110∥C.), otherwise, the heating element will be ruined. Two conducting bus bars arranged in the same direction as non-conducting threads are located in the cloth at a considerable distance from one another, thus making wire connection inconvenient.
U.S. Pat. No. 4,983,814, issued on Jan. 8, 1991 contains a description of an electric heating cloth with 1–100 kOhm/m linear electric resistance heating threads in the weft.
The heating resistive thread for this fiber also has a “shell-nucleus” structure, in which the “nucleus” consists of a nylon-, polyester- or polyolefin type (all having low melting points within 100–120° C. temperature range) synthetic fiber or high-melting polyfluoroethylene and polyamide type, and the “shell” that serves as the resistive material, is a composite that contains a polyester type polyurethane resin and a carbon filler at a mass ratio of 1:0.3 and 1:1, respectively.
The carbon filler used is industrial carbon (produced from oven or channel acetylene, and their mixtures) or graphite (natural, with a dense crystalline, flaky or amorphous structure, and artificial) at a mass ratio of 1:1.67 and 1:4 (column 8, paragraph 2 in the description) or 1:0.5 and 1:0.6 (in examples 1 and 2), respectively.
The heating resistive thread is produced by applying from one to three coats of the resistive material to the synthetic thread described above at a mass ratio of 1.7:1 and 2.8:1, respectively.
The disadvantages of this conductive thread are: requirements to apply two or three coats of the resistive material on the resistive thread “nucleus” and a heavy consumption of the resistive material even with single-coat “shell”, which increases the thread production cost. Besides, this conductive thread has two conductive bus bars that are arranged in the same direction as non-conducting threads and located in the cloth at a considerable distance from one another, thus making a wire connection inconvenient.
In view of the above, some requirements may be set forth that the conducting heating thread should meet, namely:improvement in heating properties of flexible heaters; uniform heating of the cloth surface for higher user comfort; greater operating temperature range; conveniences during the installation of the heating element and lower production cost. The market needs a better heating material that may be used in various applications and be reliable and efficient.